Detergent bars have conventionally been manufactured by one of two methods. The first of these is the so-called extrusion process in which a pre-formed composition comprising all components of the bar is typically plodded, i.e. extruded through an eye-plate, to form a continuous “rod”, and the continuous rod cut into smaller pieces of predetermined length, commonly referred to as “billets”. These billets are fed through a stamper, or alternatively, and especially in the production of low cost non-soap detergent (NSD) bars, are merely given an imprint on one or more surfaces. Such embossing or imprinting may be achieved using a die of the same dimensions as the bar surface which is hit with force, such as a mallet, or a die in the shape of a roller.
Stamping of detergent bars from an extruded billet, using a die, is carried out to give the bars a reproducible shape, smooth surface and/or to imprint a design such as a logo, trade mark or similar onto at least part of a surface of the bar. Stampers typically have a die formed in two halves each with a surface that contacts the billet during the stamping operation. These surfaces are adapted to close to a pre-set separation distance, thereby compressing the billet between the die halves to give the bar its final shape and appearance, and then separate. Excess composition is squeezed out from the die halves as they close. This is commonly referred to as the “flash”. The flash is then separated from the soap bar by transferring the bar through holes in a “deflashing plate”. Flash can account for up to 40% of the billet material and is generally recycled upstream of the extruder.
The second conventional method for the manufacture of detergent bars is casting. In casting, detergent compositions, in a heated, mobile and readily pourable state, are introduced into the top of an enclosed cavity (i.e. a mould) of the desired shape and the temperature of the composition reduced until it solidifies. The bar can then be removed by opening the mould.
Recently, a new technique for production of detergent bars has been devised. This is an injection moulding process and is disclosed in WO-A-00/53038 and WO-A-00/53039. Unlike the casting process where the composition is introduced into the mould as a readily pourable liquid prior to cooling, in the injection moulding process, the composition is introduced by application of pressure, preferably as a viscous liquid or paste. In the casting technique, the composition cools in the mould and some of the constituents form a solid structure, providing the final bar shape. With injection moulding, the composition is typically made viscous by being partially structured before it enters the mould. This can be effected either by partial pre-cooling of a hot substantially unstructured free flowing composition or by application of heat to partially destructure a solid or semi-solid composition. The partially structured composition is a non-Newtonian fluid and when pressure-injected into the mould it is thereby subjected to shear. This effect is preferably minimised. Inside the mould, upon further cooling the solidification process is completed.
As with most consumer products, the demand for diversity in product form and improvement in product qualities continues to increase. For these reasons, there is a need for robust, commercially viable processes for manufacturing good quality detergent bars comprising two or more different components (e.g. compositions or ingredients, colours etc.), present in the bar as distinct zones (e.g. stripes, swirls etc.).
Typically, the extrusion process is used to produce bars which are homogeneous with respect to the distribution of components. This is because of the nature of the process. A particulate feed is compressed and conveyed by an extruder, and subsequently extruded as a continuous rod. By the introduction of particulates of different colours to the extruder or by the injection of dye into the extruder or into the cone at the end of it, the extrusion process can be used to obtain bars with a marbled or striated effect.
The technique of coextrusion is also known, where two or more extruded process streams are passed together through a die orifice thereby forcing the two streams together. This produces a bi- or multi-colour billet. The resulting billet is then stamped to give a two-zone bar. The process streams must be sufficiently soft and sticky to ensure good mutual adhesion and yet sufficiently “solid” to allow ready conveyance and feeding of the billets to the stamper.
Up to now, the injection moulding process has not been used to produce striped, variegated or zoned bars.
The co-extrusion process generally produces zoned billets with a planar interface between the zones aligned along the axis of extrusion. On stamping, the planar surface will remain but may be angled with respect to the bar surface by appropriate orientation of the cutter and/or dies. If the two streams do not “weld” sufficiently well, this can result in cleavage planes within the bar, which can ultimately lead to bars splitting in use. Moreover, the opportunity for varying the resultant decorative effects are clearly limited.
Other problems experienced with stamping processes, are poor release of the bars from the dies and die-blocking. In die-blocking, small amounts of residual detergent left on dies builds up during continued use, which can result in visible imperfections on the bar surface. Die-blocking can also lead to poor or even non-release of the bars from the die surface.
The stamping process can also lead to undesirable distortions of, for example, the striped or variegated pattern present in the billet as the material is stamped. In addition, unlike with the manufacture of “homogeneous” bars, it is unlikely that the flash material generated in the stamping of a striped or variegated billet can be recycled back into the process, which adds considerably to the production costs.
Casting can be used to produce bars comprising different components in distinct zones of the bar. However, the variety of bar forms that can be produced is very limited and the process can be expensive to operate. Typically, a first component has to be poured into the mould and allowed to solidify before a second component is added. Clearly this is awkward, time-consuming and only allows for a limited number of bar forms to be manufactured.
In order to be castable, the formulation must be mobile and readily pourable at the elevated temperatures employed. Certain detergent formulations are viscous liquids or semi-solids at commercially realistic temperatures and therefore do not lend themselves to casting. Furthermore, in the casting process, the detergent melt tends to cool slowly and unevenly. This can lead to unwanted structural orientations and segregation of ingredients. Often some sort of active cooling system is employed in order to achieve acceptable processing times. Even when a cooling system is employed, cooling is still generally uneven through the detergent composition in the mould.
A major problem with the casting process in general, is that detergent compositions in the moulds tend to shrink as they cool. This is highly undesirable as the mould is intended to impart a distinctive shape to the bar and/or a logo of some kind. Shrinkage can take the form of dimples, wrinkles or voids, or a depression at the fill point of the bar.
One or more of the aforementioned problems is now solved by the present invention by means of which dual- or multi-zoned bars are made by an injection moulding process in which respective components are injected into a mould via separate respective orifices.